1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, printer, and facsimile machine in which an image is formed by an electrophotographic system, electrostatic recording system, and the like.
2. Related Background Art
For example, as shown in FIG. 7, an image forming apparatus which utilizes an electrophotographic system, such as a copying machine, printer, and facsimile machine, includes a drum type electrophotographic photosensitive body (hereinafter referred to a photosensitive drum) 1 as an image bearing member, and a primary charging unit 2, developing apparatus 3, transfer charging unit 4, and cleaning apparatus 5 disposed around the photosensitive drum 1. Moreover, an exposing apparatus 6 is disposed above and between the primary charging unit 2 and the developing apparatus 3.
In the image forming apparatus constituted as described above, during image formation, the photosensitive drum 1 is rotated/driven by driving means, and the surface of the photosensitive drum 1 is uniformly charged by the primary charging unit 2. Moreover, an image exposure light L provided by a laser light is applied onto the charged photosensitive drum 1 from the exposing apparatus 6, an electrostatic latent image is formed in accordance with inputted image information, and the electrostatic latent image is developed and visualized as a toner image by the developing apparatus 3.
Moreover, the toner image formed on the photosensitive drum 1 is transferred to a transferring material P such as a sheet in a transferring portion N by the transfer charging unit 4, and the transferring material P with the toner image transferred thereto is conveyed into a fixing apparatus (not shown). After the transferred toner image is fixed to the surface of the transferring material P by the fixing apparatus, the material is discharged. After transferring, a residual transferring toner adhering to the surface of the photosensitive drum 1 is removed by the cleaning apparatus 5.
Moreover, in conventional image forming apparatuses, an upper guide member 20 and a lower guide member 21 for guiding upper and lower surfaces of the transferring material P, respectively, are disposed on an upstream side of the transferring portion N, so that the transferring material P is steadily and securely guided into the transferring portion N. Additionally, the upper and lower guide members 20, 21 are constituted of insulating members, and absorb moisture particularly under a high-humidity environment, and the like. A transferring current leaking to the upper and lower guide members 20, 21 from the transfer charging unit 4 via the transferring material P having a reduced resistance is prevented from causing transferring defects such as null transfer.
Additionally, in the image forming apparatus, tip ends of the upper and lower guide members 20, 21 on a transferring portion N side are disposed at a very short interval of about 1.3 mm from the surface of the photosensitive drum 1, so that the upper and lower guide members 20, 21 steadily and securely guide the transferring material P into the transferring portion N. Therefore, since the upper and lower guide members 20, 21 are positioned in the vicinity of the transfer charging unit 4, the members easily receive a charge from the transfer charging unit 4, and are easily charged with the same polarity as that of the transferring current, that is, with a polarity opposite to that of the toner.
Therefore, a toner floating in the image forming apparatus, particularly a toner on the surface of the photosensitive drum 1 positioned immediately before the transferring portion N, and the like are electrostatically sucked by and attached to the upper and lower guide members 20, 21 (particularly, a tip end 21a closest to the photosensitive drum 1 in the lower guide member 21 disposed in the vicinity of the photosensitive drum 1). The toner adheres to the transferring material P and soils the transferring material P. This causes a disadvantage in that image quality is deteriorated.
Particularly, in a reversal developing system of the developing apparatus 3 (system for forming the toner image on the surface of the photosensitive drum 1 which holds a potential of the same polarity as that of the toner), an adhering force of the toner to the surface of the photosensitive drum 1 is relatively weak. Therefore, the toner, and the like tend to be electrostatically sucked by and easily attached to the upper guide member 20.
Therefore, in order to prevent the toner, and the like adhering to the upper and lower guide members 20, 21 from adhering to the transferring material P, the upper and lower guide members 20, 21 are constituted, for example, of electric conductive members. Moreover, a bias voltage having a polarity opposite to that of the transferring current (the same polarity as that of the toner) is applied. This method has heretofore been proposed. However, in this constitution, leakage of the transferring current into the guide members 20, 21 from the transfer charging unit 4 tends to be induced. Therefore, in conveyance of the transferring material P whose moisture has been absorbed and whose resistance has been reduced, the transferring current runs short and null transfer becomes conspicuous.
Particularly, in a contact transferring system of transferring means represented by a transferring roller, because of transferring current shortage, and reduction of a transferring voltage, transferring defects such as a spotted null (poor) transfer portion attributed to partial resistance irregularity of the transferring roller tend to become conspicuous. As a countermeasure for preventing such transferring defect, the insulating members are disposed on inner guide surfaces of the upper and lower guide members 20, 21 for guiding the transferring material P. Thereby, while a certain degree of surface potential of the guide surface is held, the transferring current can be prevented from leaking.
However, when the insulating member is disposed, the potential of the surface of the insulating member is reduced with respect to the potential of a bias applied to an electric conductive portion of the guide, and the toner adheres to a lower guide. The toner adhering to the lower guide is recovered by the transferring material P in a position in contact with the conveyed transferring material P. However, since the lower guide entirely has a flat shape, the position in contact with the transferring material P is not fixed. The position where the toner is recovered is displaced. The transferring material P contacts a position where a certain amount of toner is accumulated without contacting the transferring material P. Then, the accumulated toner shifts to the transferring material P, and sometimes becomes conspicuous as dirt on the transferring material P.
An object of the present invention is to provide an image forming apparatus in which null transfer is prevented even with respect to a transferring material having absorbed moisture, a toner, and the like are prevented from being deposited on a guide member for guiding the transferring material into a transferring portion and the transferring material is prevented from being soiled, and a satisfactory image can be obtained over a long time.
To achieve the aforementioned object, according to the present invention, there is provided an image forming apparatus comprising:
an image bearing member for bearing a developer image;
image forming means for forming the developer image on the image bearing member;
transferring means for electrostatically transferring the developer image borne on the image bearing member to a transferring material in a transferring area; and
guiding means for guiding the transferring material to the transferring area,
wherein the guiding means includes an insulating member disposed on a transferring material guide surface of the guiding means and a conductive member disposed on a surface of the guiding means opposite to the transferring material guide surface, and
a power source capable of applying a voltage having a predetermined polarity is electrically connected to the conductive member, and a projection is disposed on the surface of the insulating member.